1. Field of the Invention
The present invention relates to a thermal printer having a paper feed mechanism which is improved to provide a uniform printing quality.
2. Description of the Related Art
FIGS. 1 and 2 are a schematic side elevation and a schematic plan view of a color thermal printer of the type which is shown in, for example, the specification of Japanese Patent Application No. 62-147241 corresponding to U.S. Pat. No. 4,829,320.
Referring to these figures, a pair of first pulleys 2 is attached to both ends of a platen roller 1 so as to rotate together with the platen roller 1. A pair of timing belts 3 engaging with the pair of first pulleys 2 is stretched by a pair of second pulleys 4 and a pair of third pulleys 5. A sheet-type recording paper 6 is fed between the first pulleys 2 and an ink sheet 7 for supplying an ink. A thermal head 8 disposed in the vicinity of the first pulleys 2 is capable of transferring ink from the ink sheet 7 to the recording paper 6 in accordance with an electrical signal. A bridge 9a extending in parallel with the platen roller 1 between both timing belts 3 is provided with a clamper 9 which clamps an end of the recording paper 6 and runs in synchronization with the timing belt 3.
In the known thermal printer having the described construction, the recording paper 6 is supplied from a paper supplying mechanism (not shown) so that the leading end of the recording paper 6 is passed through the clearance between the platen roller 1 and the thermal head 8 so as to be clamped by the clamper 9. As a printing instruction is given, the thermal head 8 is pressed against the platen roller 1 with the recording paper 6 and the ink sheet 7 placed therebetween. Then, the platen roller 1 starts to rotate in the direction of arrow A so as to thermally transfer the ink from the ink sheet 7 to the recording paper 6 in accordance with the electrical signal. The ink sheet with the printing coloring agent transferred thereto is taken up by the take-up reel 7a. At the same time, the timing belt 3 is made to move in the direction of arrow B so as to convey the leading end of the recording paper clamped by the clamper 9. When the clamper 9 reaches the first pulleys 2 after passing the second pulleys 4 and the third pulleys 5, the thermal head 8 is spaced apart from the platen roller 1 to allow the clamper 9 to pass through the gap between the platen roller 1 and the thermal head 8. In this state, the ink sheet 7 is set up for the next color. When the clamper 9 is again brought to the print start position, the thermal head 8 is made to contact again with the platen roller 1 so that the printing with the second color is performed in the same manner as that described before.
This operation is repeated with third and fourth colors so that a color print is completed. The recording paper 6 is then separated from the clamper 9 and ejected.
In the above-described printing operation, when the peripheral speed of the platen roller 1 equals the running speed of the timing belt 3, the portion of the clamper 9 clamping the recording paper 6 temporarily exhibits a greater speed than the peripheral speed of the platen roller 1, because it moves along a path which has a radius greater than that of the pulleys 4, 5 during turning around these pulleys 4, 5, so that the recording paper 6 is pulled in the direction of movement. The resulting tensile force causes a variation in the speed of movement of the recording paper 6 at the printing position where the thermal head opposes the platen roller 1, resulting in an offset or mis-registration in the printing.
In order to obviate this problem, a method has been proposed in which the peripheral speed of the platen roller 1 is selected to be slightly greater than the speed of the timing belt 3. In such a method, however, slack 6a is caused in the recording paper 6 with respect to the timing belt 3 in an amount proportional to the length over which the recording paper 6 passes while thermal transfer is performed. This slack 6a absorbs any fluctuation in the movement of the recording paper 6 at the printing position at the moment at which the clamper 9 passes over the pulleys 4 or 5. Unfortunately, however, this slack 6a progressively grows large, so that when the recording paper 6 after the thermal transfer is separated from the ink sheet 7, the recording paper 6 is undesirably attracted by the ink sheet 7 by an amount corresponding to the amount of the slack 6a, with the result that the angle at which the paper is separated from the ink sheet fluctuates undesirably.
Thus, in the known thermal printer of the type in which the speed of the timing belt is less than the peripheral speed of the platen roller 1, the slack 6a caused by the difference in the speed is progressively increased to cause a variation in the angle at which the ink is separated from the ink sheet after the thermal transfer, resulting in a fluctuation in the printing quality.